The control of pressure or temperature in a boiler is typically accomplished by modulation of fuel valves and air dampers so that the energy input matches the energy output. This is normally done by monitoring the pressure within the boiler, and adjusting a modulating type motor to maintain a proper balance by regulating the valves and dampers. These type of controls normally are proportional controls. The pressure is maintained around a desired pressure or setpoint with a fixed range or deviation that is typically referred to as a bandwidth.
A typical boiler control utilizes an off state, a low fire state, a high fire state, and a modulation state between the low fire state and the high fire state. This type of modulation causes a pressure responsive device to make or break over an appropriate range to control the pressure within the boiler.
A variable control bandwidth over which the modulation occurs is achieved through mechanical adjustments. This type of control has a fixed hysteresis due to the mechanical "slop" in the system. The system is adjusted for optimum control which is typically defined as the smallest bandwidth that can be maintained without causing the control motor for the valves and dampers to be unstable or oscillate excessively. The minimum bandwidth to achieve this depends on the load type (that is the rate of change of demand) and the ability of the boiler to respond to that change. The boiler reaction time depends on the motor speed, a function referred to in the trade as a turn down ratio, and the thermal mass of the boiler. The thermal mass is defined as the amount of water in the boiler divided by the burner size in horsepower.
Different types of boilers have different types of thermal masses. These different thermal masses must be taken into account in any sophisticated control system. The different types of thermal masses can be selected from four general classes of boilers. These classes are a steam boiler with a fire tube, a steam boiler with water tubes, a hot water boiler with fire tubes, and a hot water boiler with water tubes. Each one of these four types has a different thermal mass characteristic and that characteristic must be considered in the overall control of the burner for that boiler.
In many cases optimum control is not required. Minimizing modulation, that is motor repositions, may be more important in a particular installation than the desire for an extremely tight control. The type of load response desired and the selection of the thermal mass requires in a conventional system a rather sophisticated evaluation of the system so that the bandwidth and other parameters can be properly selected to match the boiler with the load being serviced.